Flat Panel Displays are becoming increasingly more popular for applications in cellular phones, PDAs, MP3 players, automobile navigation systems, computer monitors and television. The bulk Flat Panel Displays utilized in this and other applications are Liquid Crystal Displays. The mainstream back plane technology in these displays is based on amorphous silicon (α-Si). However, for numerous advantageous reasons it is becoming more important to use polycrystalline silicon (poly-Si). The principle reasons for producing Liquid Crystal Displays using polycrystalline silicon is that Thin Film Transistors (TFTs) fabricated in polycrystalline silicon offer much better performance than TFTs fabricated in amorphous silicon. This higher performance lends itself to smaller transistors and larger aperture pixels for brighter displays, higher switching speeds beneficial in reduction of the image trail artifact associated with high speed motion in the picture, and higher current carrying capability. These attributes make possible efficient integration of the peripheral circuitry needed to drive the display electronics. As a result, the increased level and efficiency of integration lowers the manufacturing costs.
While several manufacturers already produce poly-Si displays, the current manufacturing tools mostly use “blanket” scanning in which a laser beam scans over entire surface of the amorphous silicon to uniformly crystallize amorphous silicon on the entire surface. Since Thin Film Transistors (the only devices benefiting from Si crystallization) occupy only few percent of the overall substrate area, the “blanket” approach is extremely inefficient. Due to the waste of energy and production time of unneeded high quality polysilicon, the “blanket” technique is translated into high unit costs and low throughput. Moreover, due to instability of the energy delivery to the amorphous Si surface, the “blanket” approach utilizes a high degree of shot-to-shot overlap (as high as 95%) to average out the laser beam variations which even further adds to excessive production costs.
In order to overcome the disadvantages of the “blanket” scanning for forming polycrystalline silicon layer on a substrate, the spatially selective crystallization approach has been developed which is disclosed for example in U.S. Pat. Nos. 6,979,605, 6,322,625, 6,008,101, and 5,413,958. In the disclosed systems, a laser energy is delivered selectively to predetermined regions where the TFTs are to be fabricated. In spatially selective crystallization only a fraction of the substrate is crystallized, thereby manufacturing the semiconductor devices at much lower cost than by the “blanket” crystallization technique.
A majority of the Si crystallization systems employ excimer lasers generating UV wavelengths radiation. However, the usage of excimer lasers for producing polycrystalline has several drawbacks:
(a) Instability of the excimer pulse energy which may be as high as ±6%. The laser annealing process is extremely sensitive to pulse energy stability, and therefore lasers with higher stability of the generated pulse energy are desired for manufacturing process.
(b) Another drawback of the excimer lasers is the complexity and cost of their maintenance. Excimer lasers generally require a chlorine or fluorine supply, which due to their high corrosiveness requires expensive plumbing maintenance as well as frequent fills of fresh gas due to decay of active gas species in the laser chamber during the laser operation.
Solid state lasers may be desirable for their reliability. However, solid-state lasers are usually unacceptable for the purposes of Si crystallization, since they have energy temporal non-uniformity in the range of ±6-7%.
Copper vapor laser for production of polycrystalline silicon was suggested in G. Andra, et al. “Modeling the Preparation of PC-SI Thin Films with a CU Vapor Laser”, Applied Physics, A67, pp. 513-516, 1998. In experiments which were conducted, laser pulses of high repetition rate were scanned across the α-Si film leading to highly overlapping laser spots in accordance with the “blanket” crystallization approach. By applying radiation of a copper vapor laser which emits pulses of several tenths of ns in duration comparable to that of an excimer laser, grain sizes of 450 nm have been obtained.
In contrast to the excimer laser, however, the emitted wavelength of the copper vapor laser in the visible region (511 nm and 578 nm) has a much longer absorption length in amorphous silicon, and therefore thicker films may be crystallized by copper vapor laser than by excimer laser. Additionally, a smaller energy per photon at the visible wavelength spectrum than in the UV range, enables preferential absorption by polycrystalline Si, thereby reducing heat build-up.
Despite the benefits of copper vapor laser induced Si crystallization, there has been no indication of the use of spatial selective crystallization. Therefore, it would be advantageous to apply the benefits of highly stable visible radiation produced by copper vapor laser to the principles of spatially selective approach for crystallization of amorphous silicon for production of arrays of Thin Film Transistors.